In 1907 Argentine ants (Linepithema humile) arrived in Los Angeles via a cargo ship. Within just a few years of their arrival the six-legged stowaways formed a single, massive colony—known as a supercolony—that stretched through California from south of the Mexico border to San Francisco
A liberal spraying of pesticides in the past century has done nothing to diminish the ants' numbers—L. humile infestations are the most common cause of pest control calls in southern California. The Argentine ant's takeover of coastal California is marked by small shifts in the local, native ecosystem. Populations of the coast horned lizard (Phrynosoma coronatum) have declined sharply in recent years due to the displacement of native ants the lizard depends on for food. Citrus farmers have required increasing quantities of pesticides to cope with rising numbers of aphids, scale insects and other pests that the Argentines actively protect in exchange for the sweet honeydew they produce.
In an effort to better understand and help combat the ants, a group of researchers led by Neil Tsutsui at the University of California, Berkeley, sequenced the genome of L. humile. The results were published January 31 in Proceedings of the National Academy of Sciences.
"An increasingly large number of research groups are focusing on Argentine ants because of their agricultural problems," Tsutsui says. "One of our main goals was just to provide a large resource for the community of scientists that study Argentine ant biology."
Tsutsui and colleagues found an unusually large number of chemical receptor genes that allow the ants to recognize and process different environmental chemical cues. The Argentines have 367 of these genes, compared with 174 in honeybees and 79 in mosquitoes. All ants communicate largely by "smelling" different chemicals in the environment with their antennae. Tsutsui said that identifying both the chemicals and the messages they send, researchers might be able to disrupt the Argentines' normal behavior.
"We know a lot about the behavioral processes that are regulated by these chemicals, and we know a lot about the chemicals themselves, but we don't know much about the missing links in between those two things," Tsutsui says.
Sequencing DNA is one thing; figuring out how to use that knowledge to halt the ants' spread is another. A 2009 paper in Insectes Sociaux found that Argentine ants from California, Europe and Japan formed a global supercolony. Argentines are very territorial in their South American homeland. Neighboring colonies fight each other regularly for food and nest space. These frequent brawls help keep ant numbers in check. But neighboring Argentine ant colonies that invade new locations often live side by side with nary a scuffle, so there is no in-species mechanism to keep populations down.
"If you take ants from any of those locations, they don't fight. They interact as if they are nest mates," says David Holway, an ecologist at the University of California, San Diego. "If you take Argentine ants from some other supercolony, they will fight very aggressively."
Highly adaptable street toughs
These supercolonies, however, are only one aspect of L. humile's success. Tsutsui and colleagues found a cluster of genes that allow the ants to eat a variety of foods. These genes allow the ant to metabolize the toxins frequently found in unfamiliar food. Years of evolution have honed this invader's skill at making itself at home in habitats ranging from Argentina's La Plata Basin to the cement jungles of Los Angeles to the green pastures of California's San Joaquin Valley. Frequent flooding of their native Paraná River selected for ants over time that were adapted to invading disturbed habitat and to easily moving their nests.
L. humile shows aggression not just to neighboring colonies of Argentine ants, but to any ant species. The Argentine rapidly overruns native ants in the habitats it invades. Whereas undisturbed environments might typically house upward of 10 ant species, areas that include the Argentine variety accommodate no other species.
Think of the Argentine ant as a city street tough that moves to small-town U.S., Holway said. The kid needed that big city swagger to survive on mean streets but doesn't just lose it if he moves to a mild-mannered town. The aggression and street smarts learned in an urban environment enable the transplant to establish a massive crime ring and take over the town.
What the future holds
Although Argentine ants are susceptible to some of the insecticides used in California's citrus groves, the chemicals have not made much of an impact on them. Their numbers are too large, and their supercolony spans too broad of an area. Eliminating the ants in a single household or field is temporary, and only means that it will be invaded again in the near future.
Nor is the large-scale use of pesticides the best option from an ecological standpoint. The toxic chemicals get into drinking water, not to mention rivers and oceans. Holway's lab now is looking to identify the ant's native parasites and diseases that help control L. humile in native habitats. Introducing these germs to the ants' new homes may start to keep their numbers in check.
Tsutsui himself hopes to use the new genetic data to identify how the Argentines use chemicals to lay down trails for fellow ants to follow. He also hopes to identify how L. humile learns to recognize nest mates. If Tsutsui can learn the chemical signature that identifies a colony, he can attempt to disrupt this process.
"We would be happy if, on a large scale, we can reduce the impact of these native ants so that they're only a minor pest," says Les Greenberg, an entomologist at the University of California, Riverside, who was not involved in the study. "It's probably impossible to eradicate them once they've been established."